[R-390] 5 khz difference?

[Tim Shoppa]

> Having done this, I then went through all the bands to
> > have a look at dial calibration and have discovered
> that .5-8.0 mhz, the dial reads 5 khz low on each band
> but on the higher bands it is relatively dead on. (or
> it might be said it can be set dead on using the Zero
> Adjust for the lower freqs and is actually 5khz high
> on the higher ones).  In any event, what gives here
> and why the disparity? I first thought it might be my
> 17 Mhz oscillator.  When I count the 17 mhz oscillator
> it indicates it is 4.790 khz low on all bands .5
> through 8 mhz.

That certainly points at the 17Mc oscillator. I can't blame
you for wanting it to point somewhere else, but it's
hard to avoid...

>  After 8 mhz, it changes, but I can't
> get an accurate read on its error.

Check the voltages etc. around V207, the 17Mc oscillator 
Pin 6 is supposed to get regulated +150V through R210
(to make a nominal 77V on the screen) when you are
below 8Mc. Above 8MHz it looks like it floats (DC) but
is grounded to RF. When tuned above 8Mc this
oscillator is not supposed to be oscillating, probably
the reason why you can't read it so well, but...!

Check bypass caps C275 and C326 (screen grid) and
bypass cap C328 (plate). There has been some argument
about what C275 is supposed to be, in my RF decks it
happens to have been a 0.033uF black beauty, but what
they probably *meant* to put in was a 5000pF or 3300pF
disk. The Y2K manual in different places names all 3 values,
obviously put together by a committee :-).

Check resistors R211, R210, and R207 for value and charring.
It is very common for especially the plate resistor to have gone
up in value by hundreds of percent, and this is generally
not good. There was a sub for one of these resistors to reduce
the amplitude of oscillation (Tempest it was later called) and
if you see this mod made you may want to reverse it.

Check the peaking of T207, part of the plate tank. I don't
see an official "tune-up" procedure for this but having
the plate tank out of whack isn't good for an overtone
oscillator.

>  I don't see a way to "warp" the 17 mhz crystal on but I
> am not convinced that is the problem or that there is
> only one problem.

All of my suggestions are ways of finding the problem in
something OTHER than the 17Mc crystal, since this is
the hardest crystal to come by!

It won't hurt to open the crystal oven can and make sure
that the crystal is seated reliably etc. You might even try
turning on the ovens and see if the frequency shifts the
right way! Again, grasping for straws so you don't have
to find a replacement crystal... If anyone has ordered a
"new" 17Mc crystal from one of the custom crystal makers
I'd be curious what its parameters are.

Tim.